Skeletal bone enlargement (hyperossification) was thought to only occur in endochondral and dermal bones (e.g., endoskeletons of marine tetrapods and dermal bones of certain jawless vertebrates, placoderms, and teleost fishes). However, in some arthrodiran placoderms (basal jawed vertebrates), i.e., Millerosteus minor, Compagopiscis croucheri, Eastmanosteus calliaspis, several types of bone enlargement also occur within the endoskeleton, affecting the vertebral column. Significantly, placoderm endoskeletal bone is thought to be thin-walled, ossifying only in the fibrous layer surrounding a cartilage precursor (historically called perichondral bone), rather than endochondrally/dermally. Hyperossification differs among these three species, revealing a range of internal and external bone morphologies undescribed for the placoderm endoskeleton. Thus, neural arches of M. minor are swollen in external appearance, in cross-section showing considerable deposition of layered, compact bone. In contrast, the arches of E. calliaspis are unswollen externally, with the neural spine cortex composed of the thin perichondral bone expected for placoderms. Histologically, though, the arch comprises numerous layers of perichondral bone. In C. croucheri, as in E. calliaspis, the spine and arch are unswollen externally, but the perichondral bone is thickened and compact, more similar to the condition in M. minor. Vertebral hyperossification in these taxa differs considerably in the degree and mode of cortical tissue thickening, via addition of tissue to the bone’s external (periosteal) and/or internal (endosteal) surfaces. Hyperossifcation in these arthrodires demonstrates that increases in endoskeletal bone mass are not restricted to crown-group gnathostomes (Chondrichthyes + Actinopterygii), representing a first step in the evolution of this process, involving modifications to cortical bone layers.

骨骼骨骼增大（高骨化）被认为仅发生在软骨内和真皮骨骼中（例如，海洋四足动物的内骨骼和某些无颚脊椎动物，斑节鱼和硬骨鱼类的真皮骨骼）。但是，在一些节肢动物的鞭毛虫（基颚脊椎动物）中，例如小密勒氏菌，小孔雀，东硬皮鱼（Eastmanosteus calliaspis），内骨骼内也会发生几种类型的骨增大，从而影响椎骨柱。重要的是，认为编轴器的骨骼内骨骼是薄壁的，仅在围绕软骨前体的纤维层（历史上称为软骨周围骨）中而不是在软骨内/皮肤内发生骨化。这三个物种之间的骨化过高有所不同，揭示了编轴器内骨骼未描述的一系列内部和外部骨骼形态。因此，较小的支原体的神经弓在外观上肿胀，其横截面显示出层状致密骨的大量沉积。相比之下，E。calliaspis的拱门在外部未肿胀，神经脊柱皮质由软骨膜薄骨组成，有望用于编解码器。但是，从组织学角度来看，足弓包括软骨周围骨的许多层。在C. croucheri中，如在E. calliaspis中一样，脊柱和足弓在外部未肿胀，但软骨周骨增厚且致密，与小M.。通过将组织添加到骨骼的外部（骨膜）和/或内部（骨膜）表面，这些类群中的椎体高骨化在皮质组织增厚的程度和方式上有很大不同。这些关节中的高骨化现象表明，骨骼内骨量的增加不限于冠群咬足动物（Chondrichthyes + Actinopterygii），这代表了这一过程的发展的第一步，涉及对皮质骨层的修饰。